in Composition, consists in choosing a secondary subject, having all its properties and circumstances resembling those of the principal subject, and describing the former in such a manner as to represent the latter. The principal subject is thus kept out of view, and we are left to discover it by reflection.

Nothing gives greater pleasure than an allegory, when the representative subject bears a strong analogy, in all its circumstances, to that which is represented. But most writers are unlucky in their choice, the analogy being generally so faint and obscure, as rather to puzzle than to please. Allegories, as well as metaphors and similes, are unnatural in expressing any severe passion which totally occupies the mind. For this reason, the following speech of Macbeth is justly condemned by the learned author of the Elements of Criticism:

Methought I heard a voice cry, Sleep no more! Macbeth doth murder sleep, the innocent sleep; Sleep, that knits up the ravel'd sleeve of care, The birth of each day's life, sore labour's bath, Balm of hurt minds, great Nature's second course, Chief nourisher in life's feast. Act ii. sc. 2.

But see this subject more fully treated under the article Metaphor and Allegory.